Image sensors or displays integrated in microelectronic devices currently comprise an array of photodetectors or of photoemitters formed in a semiconductor substrate. Each of the photodetectors or photoemitters is generally topped with a spectral filter intended to only transmit light for one wavelength range.
A known type of spectral filter comprises a layer made of a first material having at least one pattern made of a second material, with an optical index different from that of the first material, formed therein. The pattern extends through the entire thickness of the layer of the first material and may be periodically repeated therein. In the case of a plasmonic spectral filter, one of the first and second materials is a metal and the other one is a dielectric.
FIG. 1 is a partial reproduction of FIG. 2 of United States Patent Application Publication No. 2014/0374574 (incorporated by reference). This drawing schematically shows, in top view, a network 1 of spectral filters 3, 5, and 7 arranged above an array of photodetectors, each filter 3, 5, and 7 being arranged above of photodetector of the array. Each filter is formed from a layer of a first material having cruciform patterns made of the second material formed therein. The pattern dimensions are selected according to the range of transmitted wavelengths so that the filter has as high as possible a light transmission rate and as high as possible a light rejection ratio outside of this range. According to the transmitted wavelength range, certain dimensions of the patterns may be smaller than some hundred nanometers.
In practice, to manufacture a filter of the type of those in FIG. 1, the layer of the first material is deposited, after which, for each pattern, a hole having the shape of the pattern is etched through the layer. The second material is then deposited to fill each hole and form the corresponding pattern therein.
In the case of patterns such as crosses or stars, for example having certain dimensions smaller than some hundred nanometers, such a manufacturing method has various disadvantages, some of which at least are desired to be overcome by the present disclosure.